Doppler radar automatic signal detection apparatus

ABSTRACT

The electrical output of a noncoherent doppler radar is applied to a noise AGC system which varies the channel gain so as to maintain substantially constant the level of random noise, thereby counteracting the noise-level distorting effects of AGC in the radar I.F. amplifier. Variations in the radar output due to changes in strength or spread of clutter signals are reduced by a clutter control circuit including a self-adapting filter which automatically changes its lower-frequency cutoff skirt so as to increase the attenuation of clutter signals when the energy of the interfering clutter-signal components tends to increase. After passing through the noise AGC circuit and clutter control circuit in series, the energy of the resultant signal varies substantially only due to the presence or absence of desired moving-target-produced received signals, and is applied to an energy-responsive threshold circuit to produce automatically an output signal indicative of moving target presence or absence. Either the noise AGC circuit or the clutter control circuit can be used by itself without the other.

United States Patent Thomson et al;

[54] DOPPLER RADAR AUTOMATIC SIGNAL DETECTION APPARATUS PrimaryExaminer-Malcolm l r Attorney-Howson and l-lowson [72] Inventors: Don N.Thomson, King of Prussia; James J. Connolly Center Square; Kenneth K. 57ABSTRACT Zerger, Morrrsvrlle, all of Pa. The electrical output of anoncoherent doppler radar is ap- ASlgne1 The Magmvm p F011 Wayne, pliedto a noise AGC system which varies the channel gain so as to maintainsubstantially constant the level of random [22] Filed: Sept 10, 1969noise, thereby counteracting the noise-level distorting effects of AGCin the radar I.F. amplifier. Variations in the radar out- PP 856,620 putdue to changes in strength or spread of clutter signals are reduced by aclutter control circuit including a self-adapting filter whichautomatically changes its lower-frequency cutoff (gill ..343/7 7 skin soas to increase the attenuation of clutter Signals when the energy of theinterfering clutter-signal components tends [58] Field of Search..343/7.7 to increase. After p g through the noise AGC circuit andclutter control circuit in series, the energy of the resultant [56]References Cited signal varies substantially only due to the presence orabsence UNITED STATES PATENTS of desired moving-target-produced receivedsignals, and is applied to an energy-responsive threshold circuit toproduce au- 3,078,458 2/1963 Stemberg 343/ 7 7 tomatically an outputsignal indicative of moving target 3361468 8/1966 "343/ 7 7 presence orabsence. Either the noise AGC circuit or the i clutter control circuitcan be used by itself without the other. 1r pa c 3,465,336 9/ i969Fishbein et al ..343/7 7 4 Claims, 4 Drawing Figures /2Z 5 aiazw 24 a aa i l mam-awn: r i 44 "Mi/m Mmumrm rm/yawn? ....c U? i $54622? /V Iaway/rs 40 16 I J2 J I F! i pzrzcm? MIXER I 1 I T 1 if I 04w: We I23275- aa/ fiia ra/e E l Mil/W44 a I 1% L7,, v i pwr/iz i .J

/2 T 0 55 [iii/ r 2225322 Ll/PLU/f 1 I PAIENIEDFEB 15 m2 SPEC 7F '470073 0760) #0495 in??? a: 07727? 14905 575540)" SHEET 2 OF 2 INVENTORSZDON N. THOMSON JAMES J. CONNOLLY KENNETH K. ZEIGER BWK/ ATTYS.

DOPPLER RADAR AUTOMATIC SIGNAL DETECTION APPARATUS BACKGROUND OF THEINVENTION There are a variety of applications in which it would bedesirable to provide automatic detection of the presence or absence of adesired signal in the presence of interfering signals of varyingstrength. When the interfering signals are of substantial magnitudecompared with the desired signal and also vary in level, the concept ofderiving a signal indicative of the energy of the combined signals overpredetermined time intervals and comparing the resultantenergy-representing signal with a threshold cannot be utilized in manycases with a satisfactory degree of reliability and sensitivity. This isbecause the combined signal energy, and hence the energyrepresentingsignal, will vary not only with the presence or absence of thedesiredsignal but also with the above-mentioned variations in level ofinterfering signals. In such a situation, if the threshold level is setsufficiently high that it will not be exceeded due to variations in theinterfering signals, the system will either not respond to the presenceof a desired signal or will only respond to such a high level of thelatter signal that the resultant automatic detection is undesirablyinsensitive. On the other hand, if the threshold level is set too lowthe system will be actuated in response to changes in energy of theinterference signals, producing false indications of the presence of thedesired signal.

One particular application in which this problem arises is in so-callednoncoherent doppler radar systems, with particular reference to whichthe invention will be described in detail hereinafter. In typical formsof such apparatus, pulses of microwave energy from a transmitter areradiated by a directional antenna toward a region in which the presenceof reflective moving targets is to be detected. Reflections from bothmoving and stationary targets are received by the radar, amplified, andsubjected to a range-detecting operation which enables categorization ofthe reflections with respect to distance from the transmitter. Thosereflections produced by objects moving with respect to other'fixed orreference objects, such as the ground, are shifted in frequency due tothe doppler effect, and are enhanced in the combined signal byappropriate filtering devices. The reflected signals received from thestationary or reference objects are commonly referred to as clutter"signals and are relatively easy to separate from the desiredmoving-target signals when the target-object velocities are large andthe frequency differences between moving-target signals and cluttersignals correspondingly large. However, for low velocities of targetssubstantial overlap between frequency components of the clutter signalsand of the moving target signals may occur, rendering discriminationbetween them on the basis of frequency either difficult or impossible.That is, if frequency discrimination between clutter and moving-targetsignals is to be provided by positioning of the lower-frequency skirt ofa high-pass or band-pass filter, the skirt can be positioned high enoughin frequency to reject all clutter, but in this event slowly movingtargets will not be detected; on the other hand, if the filter skirt ispositioned too low in frequency, low-velocity moving target signals willbe passed by the filter but will be obscured by clutter signals havingsimilar frequency components. Ordinarily, therefore, the lower-frequencyfilter skirt will be positioned just sufficiently high in frequency toprevent frequent passage of clutter signals of a strength such as toobscure the desired moving-target signals, and no higher. Moving targetshaving velocities below that corresponding to the selected filter-skirtposition will then not be detected by such a system.

lf the clutter signals are constant in strength and frequency spread, itwould still be possible to detect the presence or absence of desiredmoving-target signals even though the filter characteristics were suchas to permit substantial overlapping of clutter signals into theselected pass band. This could be accomplished by means of circuitswhich would produce a signal representing the energy of the combinedclutter and movingtarget signals during any particular pulse-reflectioninterval; if a target signal were present, the energy-representingsignal would rise above a reference threshold and produce a targetpresent" signal, and otherwise would produce a no target" signal.However, the amount of energy due to clutter signal passing through thefilter varies in response to changes in both the clutter signal strengthand the frequency-spectrum spread of the clutter signal, which readilyoccur due to such factors as minor movement of vegetation anddifferences or variations in reflectivity of nominally-stationaryreflecting objects. Such variations will have the same effect as thepresence of a desired signal, and will render unreliable the finaloutput indication of target presence. Accordingly, using such prior arttechniques it would be necessary to position the filter skirtsufficiently high in frequency to attenuate substantially completely allclutter-frequency components, thereby inherently and undesirablypreventing the passage and detection of target signals due toslowly-moving objects.

A further problem arises with respect to the random noise unavoidablyintroduced into the signal channel in any radar system receiver. Suchnoise fluctuates randomly in amplitude and hence makes it difficult orimpossible to detect a small desired signal in the presence of suchnoise by ordinary amplitude detection. If the random noise levelremained substantially constant, it would again be possible to detectthe presence or absence of a desired signal by sensing changes in theenergy of the combined desired signal and noise. However, in mostpractical systems the noise level varies unpredictably. For example, thereceiver amplifier of noncoherent doppler radars is usually providedwith automatic gain control (AGC) operating on the total received signal(usually dominated by clutter) to prevent overloading of the output ofthe amplifier, and such AGC normally operates inherently to produce widevariations in the level of random noise at the amplifier output. Thisagain will produce the above-mentioned difficulty in attempting toprovide an appropriate and reliable setting of any threshold devicesupplied with a signal representative of the energy of combined signaland random-noise energy and intended to discriminate between thepresence and absence of the desired moving-target signals.

In connection with the above-mentioned use of an AGC system in thereceiver amplifier, it will be appreciated that the received signalsfrom moving and fixed objects, while largely separate in the frequencydomain, are superimposed in amplitude in the time domain. Accordingly,if strong received clutter-signals cause overload of a later stage inthe amplifier, the desired moving-target signal variations areessentially lost, and it is therefore important in such circumstancesautomatically to reduce the gain of the amplifier so that the signals atthe output of the amplifier are in the linear-amplification regionthereof, or at least below the level of strong overload and limiting.

In some applications which can be envisioned, the variations due to thechanging level of random noise may be impor tant and the variations dueto clutter changes unimportant, and vice versa for other systems.However, in usual types of practical noncoherent doppler radar systems,both types of variation will occur and will present severe limitationson the ability to provide reliable automatic detection of weak signalsfrom moving targets over a wide range of velocities.

Accordingly it is an object of the invention to provide new and usefulapparatus for sensing the presence or absence of desired signals in thepresence of interfering signals.

Another object is to provide such apparatus capable of detecting weakdesired signals in the presence of random noise of varying level.

A further object is to provide such detection which is automatic, inthat it automatically produces a signal representative of said absenceor presence of desired signals.

Another object is to provide new and useful apparatus for automaticallydetecting desired signals over a wide range of frequencies despitevarying degrees of overlap of undesired interfering frequency componentsinto said range.

It is also an object to increase the range of velocities of movingtargets for which moving-target-indicating signals may be automaticallydetected in a noncoherent doppler radar system.

A further object is to provide a new and useful noncoherent dopplerradar system with means for automatically detecting the presence orabsence of desired moving target signals.

Another object is to provide such an improved system in which areceiver-amplifier employing automatic gain control is employed.

It is also an object to provide the latter type of system in whichreceived clutter signals vary with respect to strength and frequencyspread, and in which the detection of lowervelocity moving targets isenhanced.

Another object is to provide a new and useful noncoherent doppler radarsystem for automatically detecting the presence or absence of movingtarget objects despite the presence of varying and substantial levels ofclutter signal and random noise.

SUMMARY OF INVENTION These and other objects of the invention areachieved by the provision of an automatic signal-detection system inwhich an energy-threshold comparator is used to detect changes in energyof a combined signal comprising desired signal components extendingthrough a predetermined range, interfering signal components of varyingenergy overlapping into one end of said range, and random noiseextending through said range, said system comprising a noise AGC systemwhich selectively responds to, and acts upon, the varying random-noiselevel to return it to a substantially constant level prior toapplication of the combined noise and signal to the energy-thresholdcomparator; said system also comprising a clutter-control circuitincluding a self-adapting filter (SAF) through which the combined-signalcomponents are passed prior to application to the energy-thresholdcomparator, the frequency position of the skirt of the SAF beingautomatically varied so as to reject or attenuate said interferingcomponents more strongly when the energy thereof tends to increase.Preferably the noise AGC and the SAP clutter-control circuit areconnected in series in the signal channel so that the effects ofvariable random-noise level and variable interfering-signal strength andfrequency spread are all mitigated, and the threshold comparator therebyenabled to operate sensitively and reliably.

In the preferred embodiment, a noncoherent doppler radar having theusual audio bandpass filter for separating movingtarget frequencycomponents from clutter components supplies its output to the input of anoise AGC circuit comprising variable-gain means, a noise filterconnected to the output of the variable-gain means to select noisecomponents outside the frequency range normally occupied by eitherdesired target signals or clutter signals, and a detector and low-passfilter combination for producing a control signal indicative of thelevel of noise in the output of the variable-gain means and for applyingthe control signal to the variable-gain device in the polarity tomaintain substantially constant the noise level at the output of thevariable-gain device. The output of the variable-gain device is alsopassed through a self-adapting filter, the lower-frequency cutoff skirtof which is varied in frequency position automatically so as to moveupward in frequency and reject the clutter-frequency components morestrongly when they become stronger or more widely spread in frequency soas to tend to introduce greater contamination into the desired signal.In one preferred embodiment, automatic control of the SAF may beprovided by means of a filter responsive to the input to the SAFselectively to pass frequency components in the frequency range of thecontaminating clutter components, a detector and low-pass filterarrangement supplied with the output of the latter filter for producinga signal representative of the energy of the contaminating clutterfrequency components, and an appropriate circuit arrangement responsiveto the control signal for varying the lower frequency cutoff skirt ofthe SAP in the direction to oppose increases in clutter contamination.Alternatively, the output of the SAF can be fed back to control theposition of its lower-frequency skirt, so as to maintain the cluttercontamination energy at or below a predetermined value by servo-feedbackcontrol. The output of the SAF is supplied to an energy-responsivethreshold comparison circuit, which typically may comprise an integratorfollowed by a voltage comparator, by means of which the processedcombined signal is compared with a variable threshold to operate analarm when the threshold is exceeded; alternatively, the output of theSAF may be supplied to a probability ratio sequential detector (PRSD)containing the usual two thresholding devices and operating in a mannernow well known in the art.

In some applications either the noise AGC system or the clutter controlcircuit of the invention may be used by itself, rather than in tandemcombination with the other.

Because the energy of frequency components due to random'noise orclutter interference is maintained substantially constant at the inputto the integrator and thresholding circuit, any change in the energy ofthe combined signal, and in the output of the integrator, can properlybe ascribed to the presence of the desired moving-target signals; or atleast the probability of thereby obtaining a correct decision as to thepresence of such target objects is enhanced. The sensitivity andreliability of the system is thereby improved or, alternatively, thepower requirements of the equipment can be reduced for the samesensitivity and reliability.

BRIEF DESCRIPTION OF FIGURES These and other objects and features of theinvention will be more readily understood from a consideration of thefollowing detailed description, taken in connection with theaccompanying drawings, in which:

FIG. 1 is a block diagram illustrating a conventional noncoherentdoppler radar system to which has been added an automatic detectionsystem in accordance with the invention;

FIG. 2 comprises a series of graphical representations plotted to acommon frequency scale, to which reference will be made in explainingcertain characteristics and operations of the system of the invention;and

FIGS. 3 and 4 are block diagrams illustrating alternative arrangementsin accordance with the invention.

DESCRIPTION OF SPECIFIC EMBODIMENTS Referring to the embodiment of theinvention shown in FIG. 1 by way of example only, a noncoherent dopplerradar 10 supplies output signals to the series combination of a noiseAGC circuit 12 and a clutter control circuit 14, the output of thelatter circuit being supplied in parallel to energy-threshold detectioncircuits l8 and 20.

The noncoherent doppler radar 10 may be entirely conventional. In thisexample it comprises a pulse repetition frequency (PRF) generator 22supplying a modulator 24 for modulating the transmitter 26 to cause itto transmit time-spaced pulses of microwave energy into space through aT-R duplexer 28 and a suitable directive antenna 30. Reflected microwaveenergy is received by the antenna 30 and passed through the T-R duplexer28 to the usual microwave mixer 32, to which local-oscillator signalsare also supplied in the usual way from a local oscillator 34, which inturn may be provided with appropriate automatic-frequency-eontrolcircuits (not shown) as is usual in such equipment. The output of mixer32 is amplified in an intermediate-frequency (I.F.) amplifier 38 andthen supplied to a detector 40, which derives video voltage variationscorresponding to the envelope of the received, reflected, microwavesignals. The video signal from detector 40 is passed through rangingcircuits 42, which are also supplied with timing impulses generated bythe timing circuits M in response to synchronizing signals from PRFgenerator 22.

Ranging circuits 42 in this example may comprise a gate circuit whichpermits passage only of signals occuring during a short gate-pulseinterval following each transmitted pulse by a controlledly-variabledelay. The delay of the gating pulses with respect to transmitter pulsesis variable in response to control signals applied thereto from a rangestrobe circuit 46, the internal operations of which are timed by signalssupplied from the timing circuits 44. Range strobe circuit 46 suppliesthe ranging circuits 42 with a series of gating pulses differentlydelayed with respect to transmitter pulses according to a predeterminedpattern. For example, it may first produce gating pulses delayed withrespect to transmitter pulses by a predetermined short delay,corresponding to a short target range, then after a number of suchgating pulses the gate pulse delay may be automatically changed to aslightly larger value corresponding to a slightly greater range, and soon, until the complete range interval of interest has been explored inthis manner. In addition, range strobe circuit 46 is preferably providedwith a manual range control input 48 which, when actuated, terminatesthe automatic range strobing action and causes the delay of the gatingpulses to be controllably adjustable by hand. The manual range control48 is preferably calibrated so that the operator can readily determinevisually the range being investigated for a given adjustment of thecontrol. Accordingly, output from the ranging circuits 42 consistssubstantially only of random noise plus frequency components due toreflections from objects in the range bin corresponding to the delay ofthe contemporaneous gating pulse applied to the ranging circuits.

Output from the ranging circuits is passed through a bandpass filter Fto an audio transducer 52, which may be a simple pair of earphones, theacoustic output of which is monitored aurally by the operator.

Briefly, in the known operations of the radar transmitted pulses arereflected from fixed reference objects such as the ground and objectsattached thereto, including for example various forms of vegetation, aswell as from desired movingtarget objects. For example, in one intendeduse of such ap paratus for military surveillance in ground combat areasthe radar is of a compact, lightweight low-power form portable by anoperator in the field, and the antenna is small and manually orientableby the operator. Desired moving-target objects may, for example,comprise vehicles moving on the ground. The reflections returned to theantenna 30 comprise the fixedobject reflections or clutter signals,combined with the desired reflections from moving-target objects whichhave microwave frequencies differing from those of the clutter signalsdue to a doppler shift proportional to the velocity of the moving-targetobjects. Since the transmitter pulse comprises a spectrum of frequencycomponents clustered around harmonics of the pulse repetition rate, thereflections contain corresponding frequency components, except that hosereflected from moving target objects are shifted substantially infrequency and those from fixed objects are slightly dispersed infrequency to a variable extent depending upon small motions of thereference objects, such as the motion of vegetation in the presence ofstrong wind. As to strength of reflections, the more remote the targetobject and the smaller it is, the weaker the reflection. In the case ofreflections from ground, reflections at near ranges are very strong, andtheir strength falls off rapidly with increasing range.

The above-described combined received signal, after being heterodyned tointermediate frequency by mixer 32, passes through the LP. amplifier 38,which has a passband substantially equal to the inverse of thetransmitted pulse width and therefore passes frequency componentsdiffering from the center intermediate frequency by relatively highmultiples of the pulse repetition frequency, so that the pulse shape islarge lypreserved in the video output of detector 40. As mentionedabove, the instantaneous condition of the range strobe circuit 46 or thesetting of manual range control 48 determines the particular incrementof range which is being examined and which is being represented by theoutput of the ranging circuits 42. The frequency components in theoutput of the ranging circuits include those produced by heterodyning,or beating-together, of the dopplenshifted signals and the cluttersignals from reference objects.

At A of FIG. 2 there are represented graphically and schematically thelower-frequency portion of the spectrum of the clutter signal C, and ofthe spectrum of the desired targetreflected signals S which have beenshifted by the doppler shift 5 f In addition there is shown theeffective or RMS randomnoise level N due to such factors asthermally-generated receiver noise, which is assumed to have asubstantially uniform spectral density. For more slowly-moving targetsthe signal S will have lower frequency components and a spectrumposition closer to the clutter signals C, and for higher-velocitytargets will be shifted farther to the right toward higher frequencies.The broken curved line illustrates the clutter signal C when itsspectrum has been broadened, which can occur randomly and unpredictablyas mentioned above due to such factors as wind. What is desired is todetect the presence or absence of the moving-target signal S in thepresence of the clutter signal C and of the random noise N At B of FIG.2 there is shown an idealized bandpass characteristic for the filter F,,which as shown has a passband positioned to embrace the signal S and asubstantial range on each side thereof so as to accommodate differentvelocities of target motion, while rejecting at least a substantialportion of the clutter-signal spectrum. The result is that shown at C ofFIG. 2, which represents an idealized version of the frequency spectraat the output of filter F As shown, the output of filter F, comprisesthe desired signal S, the noise N extending throughout the passband ofthe filter, and the portion of one edge or tail of the clutter spectrumC.

in order better to understand the operation of the system, somerepresentative typical numerical values will be helpful. As an example,the pulse repetition frequency may be 5,000 Hz. when X-band microwavesare used, and the passband of filter F may extend from about 30 Hz. toabout 2,000 Hz.

The output of filter F supplied to audio transducer 52, is listened-toby the operator, who has been trained to detect with reasonablereliability the presence or absence of the audible tone due to thesignal 5 despite substantial interference from the noise N and from thetails of the clutter signal C. When the energy of interfering signals inthe passband of filter F, increases, a point will be reached at whichthe operator will be unable reliably to detect the presence or absenceof the audible tone due to signal S. Below this point, however,reasonably accurate detection can be accomplished, with sufficientconcentration by the operator. With the range strobe circuit 46 in itsautomatic strobe operation, the operator will normally listen until hedetects a signal due to a moving target, and then actuate and operatethe manual range control 48 to hold the range gate at a positioncorresponding to the range of the target, thereby to keep the targetunder surveillance, i.e., to track" it manually.

One prime difficulty with this prior-art system is that it requirescontinuous intense concentration by the operator, which not onlyproduces fatigue and resultant inefficient monitoring and detection bythe operator, but also requires all of his attention while in the fieldwhen at least occasional attention to other activities may be ofsubstantial or vital importance. if, on the other hand, an automaticdetection system can be provided which will produce an alarm signal inresponse to the presence of a moving target object, the operator mayrest or direct his attention elsewhere until the alarm occurs, at whichtime he may direct his attention to aural surveillance and tracking of atarget producing the alarm.

One form of automatic detection apparatus shown in FIG. 1 is theenergy-threshold detection circuit 18, comprising a rectifier 59, anintegrator 60, a voltage comparator 62 supplied with the output of theintegrator and with a threshold voltage from variable-threshold source64, and an alarm 66 supplied from the comparator. The rectifier 59 issupplied with the output of filter F, by way of the noise AGC circuit 12and the clutter control circuit 14. Integrator 60 may be of theconventional RC type or of the integrate-and-dump type in which dumpingor discharging is provided in response to a timing pulse supplied fromtiming circuits 44 just prior to each change in the range bin positionduring range strobing, by way of an appropriate interconnection (notshown). The function of integrator 60 is to produce an output voltagewhich varies in proportion to the energy of the signal supplied thereto.Accordingly, if a desired moving-target signal is present the integrator will produce a larger output voltage than if that signal isabsent, provided that interfering clutter signals and noise remainconstant.

The comparator 62 receives the energy-representing output voltage ofintegrator 60 and compares it with a variable voltage from thresholdsource 64. If the energy-representing voltage exceeds the thresholdvoltage, the comparator delivers an output signal to operate alarm 66,which may be visual or aural. The voltage from the variable thresholdsource 64 is ideally set just sufficiently high that the alarm 66 is notoperated in the absence of the desired moving-target signal but isoperated when moving-target signals are applied to the integrator 60.Also shown in FIG. 1 as being supplied with the output of the cluttercontrol circuit is the energy-threshold detection circuit 20, comprisingrectifier 67 and a probabilityratio-sequential-detector (PRSD) andcontrol circuit 68 for controlling operation of alarm 69 and of radarranging and display circuit 72. In some cases only one of theenergy-threshold detection circuits is and 20 will be utilized, but bothmay be used simultaneously to advantage in some applications. Suitableapparatus for use in the PRSD and control 68 will be apparent to oneskilled in the art from a consideration of US. Pat. No. 3,253,277 of G.W. Preston et al., issued May 24, 1966 and US. Pat. No. 3,271,762 of K.K. Zeiger, issued Sept. 6, 1966, both included herein by reference. Ingeneral, the PRSD normally includes a pulse-to-pulse integrator suppliedwith the output of the rectifier, and a parallel combination of a lowerthreshold device and an upper threshold device both supplied with theoutput of the integrator. After a number of transmitted pulses haveoccurred, either the upper threshold device or the lower thresholddevice, or neither, will have been operated by the received signal. Inone preferred form, if neither threshold is exceeded after apredetermined number of transmitted pulses a synthetic indication isgenerated that the upper threshold has been exceeded.

In the FIG. 1 arrangement, the alarm output of the PRSD and controlcircuit 68 is supplied to alarm 69, for example a lamp. Another output,supplied to the radar ranging and display unit 72, can be used toprovide a visual indication of the presence of a moving target at aparticular range. Another output at line 74 comprises a control output,which may be utilized to control antenna scanning in cases in which suchscanning is automatically controllable, or to control the generation oftransmitter pulses or the progress of range exploration by the rangestrobe circuit 46. However, the particular construction, operation anduse of the PRSD device is not critical in connection with the presentinvention. The significant fact with respect to the PRSD for the presentpurpose is that it utilizes threshold detection of the energy in thereceived signal applied thereto.

Accordingly, both in the case of the PRSD-type of energythresholddetection apparatus 20 and in the case of the more elementaryenergy-threshold detection apparatus 18, the reliability of indicationof the presence or absence of desired moving-target signals issubstantially reduced if the energy due to clutter signals and/or randomnoise also varies substantially and unpredictably. It will now bepointed out wherein such undesired interfering variations arise in theoutput of radar l0, and how they may produce unreliable outputindications in the absence of the noise AGC circuit 12 and the cluttercontrol circuit 14 of the invention.

Radar conventionally employs an AGC system comprising low-pass filter 80and AGC amplifier 82 serving to feed back signals derived from theoutput of ranging circuits 42 to a gain-control terminal of the LF.amplifier 36. The purpose of such AGC is to prevent such overload of[.F. amplifier 38 as would tend to reduce or eliminate variations in theoutput of the l.F. amplifier due to the desired moving-target signals.For this purpose low-pass filter 80 preferably has a time constant aslong as possible consistent with remaining less than the period duringwhich the ranging circuits monitor a particular range bin. If theclutter signal power were reasonably constant, the fact that it isnormally much greater than the moving-target signal would cause it todominate the AGC circuit and maintain the I.F. amplifier gainsubstantially constant in a given application. However, instead, thestrength of the clutter signal varies greatly with range, and as therange strobe circuit 46 strobes outwardly in range the clutter signaldecreases from a very high value to a relatively low value, and the gainof the [.F. amplifier 38 is thereby automatically varied from arelatively low value to a very high value.

The effect of this gain variation on the noise from the I.F. amplifieris illustrated at D and E of FIG. 2. D represents the case for a strongclutter signal and E that for a weak clutter signal in the input to theIF. amplifier. Because of the AGC action, the clutter signal at theoutput of the IF. amplifier is nearly the same in the two cases, but theeffective noise level N is much higher when the clutter is small. Thischange in noise level corresponds to a very substantial difference inenergy of the spectra for the two conditions shown at D and E, eventhough no moving-target signal is present. From the foregoing it will beapparentthat this change in energy will produce false output indicationsfrom the energy-threshold circuits unless the thresholds are set so highthat this cannot occur, in which case weak desired signals will not bedetected by the threshold devices.

It is this difficulty which is overcome in accordance with the inventionby the use of the noise AGC circuit 12. Output from filter F, issupplied to variable-gain amplifier 86, the output of which is suppliedto a filter F having a frequency-response characteristic such as to passsubstantially only noise frequency components, free of clutter signalsand moving-target signals. As shown at F of FIG. 2, a suitable positionfor the bandpass characteristic of F is at the extreme higher-frequencyedge of the bandpass characteristic of filter F With this position, theclutter signals do not extend into the passband of F and, whileincreases in velocity of moving targets tends to shift the moving-targetsignal higher in frequency, the velocities of moving targets encounteredin use of the equipment are not sufficiently high to introducesubstantial interference in the passband of F Accordingly, the output offilter F varies only with, and is a measure of, the random-noise level NTo derive therefrom a control signal varying with the latter level, theoutput of F is passed through a detector 88 and a low-pass filter 90 toproduce a direct-voltage control signal varying in accordance withvariations in the noise level of N an amplifier and adjustable thresholdcircuit 91 can be included in this feedback circuit if desired. Thelatter control signal is applied to the gain-control terminal ofvariable-gain amplifier 86 to vary its gain in the direction to maintainthe noise level at the output of amplifier 86 substantially constant ata level determined by the adjustment of the threshold in amplifier andthreshold 91. In effect, the uniform noise level which was distorted bythe AGC in the radar 10 is thereby restored to a substantially uniformlevel. Accordingly, the combined signal entering into theenergy-threshold detection circuits l8 and 20 does not contain theabove-described noise energy variations, and the threshold levelstherein can therefore be set for more reliable and sensitive detection.

As an example, utilizing X-band microwave transmissions, doppler shiftsf greater than 1.7 kHz. can be produced only by targets havingvelocities in excess of 50 knots. which is highly unlikely for anytargets in a ground combat environment. Bandpass filter F may then beselected so that its center frequency is at about 2 kHz. and itsresponse less than halfmaximum at about 1.7 kHz. Apparatus for producingnarrow passband filters is so well known that it is unnecessary to showand describe specific apparatus for this function. As an example, only,typically the desired bandpass characteristic may be provided by twoinductance-capacitance circuits connected as the loads for twosuccessive amplifying stages.

Since the bandpass filter F has removed the DC component of the receivedand amplified signals, the noise signals at the output of filter F arein alternating form. Detector 88 serves to rectify this signal, as byhalf-wave or full-wave rectification. Such rectifying circuits beingwell known in the art, it is unnecessary to show or describe them indetail therein. Similarly, the low-pass filter 90 may take any of avariety of known forms, such as a series ofresistancecapacitancesections in tandem serving in effect to average thedetected noise signal, thereby to produce the desired slowly-varying DCcontrol voltage for variation of the gain of variable-gain amplifier 86.Variable-gain amplifiers suitable for such purposes are also well knownin the art, and need not be exemplified in detail; the gain-varyingmechanism may, for example, comprise a voltage divider arrangement, oneof the resistances of the voltage divider being shunted by afield-efiect transistor whose conductivity is varied in accordance withthe control signal from low-pass filter 90. Many suitable forms ofimplementation of this variable-gain device will occur to one skilled inthe art.

The overall effect of the noise AGC circuit is illustrated at G and H ofFIG. 2. G and H of FIG. 2 show the output of the noise AGC circuit forlarge clutter and small clutter respectivcly. Unlike the situation shownat D and E of FIG. 2, in this case the effective noise level N remainssubstantially constant despite large changes in clutter signal.

Turning now to the problem created by variations in clutterinterference, as illustrated for example at G and H of FIG. 2

the clutter signals supplied to clutter control circuit 14 may varygreatly in strength, and as shown at A and lof FIG. 2, the frequencyspread of the clutter may also vary unpredictably due to differences interrain and in the motions of parts thereof. For this reason the energyof the contaminating components due to clutter signals (i.e., thosewhich overlap into the lower-frequency end of the passband of filter F,)also varies unpredictably and, unless some special provision is made,will render the energy-threshold apparatus less reliable or lesssensitive. Comparing the waveforms at B and C of FIG. 2, it will be seenthat it would be possible to design the filter F, so that itslower-frequency skirt occurs at a higher-frequency position, such thatthe desired moving-target signal S is passed and the clutter signal Csubstantially completely rejected. In this event the clutter energytransmitted by filter F, would be so small that variations in thisenergy would have little harmful effect. However, if this is done thenslower moving targets will produce frequency components so low infrequency as to be attenuated or eliminated by filter F,, and hence willnot be detected. Such an arrangement would therefore mean that, in orderto obtain high reliability and sensitivity, detection of slower movingtargets would have to be sacrificed at all times. Clutter controlcircuit 14 mitigates this problem in the manner now to be described.

At J of FIG. 2 there is represented the spectral characteristic ofself-adjusting filter F in idealized form. The characteristic is that ofa high-pass filter having a lowerfrequency skirt which is automaticallyvaried between the lower-frequency position shown in full line and thethree higher-frequency positions shown in broken line. The operation ofthe clutter control circuit is such that when the cluttersignal energycontaminating the output of filter F, is small, for example as shown atH of FIG. 2, the lower-frequency skirt of filter F occupies the positionshown in full line. This is its lowest-frequency position, and willpermit passage of desired signals from relatively-slowly moving targetobjects so as to enable their detection by the system, and will rejectenough of the weak clutter signal to prevent serious interferencethereby. When the clutter-signal energy in the passband of filter F,increases, due to increasing clutter strength or frequency spread, asshown for example at I of FIG. 2, the lower-frequency skirt of filter F,automatically moves upward in frequency so as to reject the increasedclutter signal contamination, or greatly attenuate it. In this way thereliability and sensitivity of the automatic detection system ispreserved,

although the very slowly moving target objects cannot under the latterconditions be detected with reliability. The operational advantage overthe prior-art arrangement previously described lies in the fact that thevery slowly moving targets can be detected reliably when the cluttercontamination is small.

In the embodiment illustrated in FIG. 1, the clutter control circuit 14operates to sense the energy of contaminating clutter signals in theoutput of filter F, and to switch the position of the lower frequencycutoff skirt of filter F, to the appropriate one of its four positionsrepresented at J of FIG. 2. Accordingly the low-pass filter F, isconnected to the input line for filter F and has a higher-frequencycutoff approximately the same as the lowest frequency position of thelowerfrequency skirt of filter F Filter F, therefore selectively passesfrequency components of the contaminating clutter signal lying within apredetermined frequency band at the lower end of the passband of filterF,. The output of filter F, is passed through a detector 92, again-control 93 and a low-pass filter 94, corresponding elements ofwhich may be similar to corresponding elements of the noise AGC circuit,although they also may take other conventional forms. These elementsserve to produce at the output of the low-pass filter 94 aslowly-varying direct voltage varying in accordance with the energy ofthe clutter signals contaminating the desired moving-target signals. Inthis embodiment the latter control signal is applied to the input ofanalog-to-digital converter and logic circuit 96, which has four outputlines such as 98. Analog-lodigital converter and logic circuit 96produces an output signal only on a particular one of its output lines98 when the value of the control signal from the low-pass filter 94 isbelow a first level, produces a signal only on a second differentpredetermined one of its output lines 98 when the control signal isbetween the latter first level and a higher second level, produces anoutput only on a predetermined third different one of its output lines98 when the control signal is between said second level and a thirdhigher level, and produces an output signal only on the fourth of theoutput lines 98 when the control signal is above the third level. Eachof the four output lines such as 98 is connected to a different one offour corresponding associated control terminals such as 99 of filter FSuitable circuits for producing this simple quantizing and digitizingoperation are well known in the computer and logic art and hence neednot be shown or described herein in detail.

One preferred form of the filter F is one having a plurality of filtersections, four of which are electrically connectable into the filtercircuit in response to closing of appropriate electronic switches. Eachof the four controllably-connectable filter sections will produce adifferent one of the lowerfrequency skirt positions shown at J of FIG.2. Accordingly, depending upon the energy of the contaminating cluttersignals passing through filter F,, a particular one of the controlterminals 99 will be supplied with a signal and will respond to connectthat one of the four filter sections which provides the lower frequencyskirt position appropriate for the then-existing degree of cluttercontamination. If desired, instead of electronically substitutingdifferent filter sections in filter F filter sections may be added orremoved to produce the progressive change in lower frequency skirtposition. That is, for the lowest position of the lower-frequency skirtone such filter section may be connected, for the next-higher frequencyposition a second filter section may be added, and so on. Since thesearrangements involve merely the electronic switching of filter sectionsin and out of a filter circuit in response to signals applied to thecontrol terminals 99, the circuit can readily be implemented by oneskilled in the art in a variety of ways and hence need not be shown ordescribed in further detail herein.

Preferably the switching operations described with respect to theclutter control circuit [4 occur rapidly, in a small fraction of thetime over which integrated observations are made by the energy-thresholdequipment.

Among other variants of the clutter control circuit 14 which will occurto one skilled in the art, one particularly useful type supplies theinput terminals of filter P with the output of filter F compares theoutput signal from filter F, with a reference threshold, andcontinuously changes the lower-frequency skirt position of filter F soas to maintain the fed-back signals substantially equal to the referencethreshold. This control can be implemented by constructing F;, withactive or passive filter sections and controlling the resistivecomponent thereof electronically, as by use of field-effect transistors.In the design of such feedback clutter-control systems using continuousvariation of the lower-frequency skirt of F;,, the selection of theselectivity of the latter skirt and of the higher-frequency skirt of Fis significant for best operation. Specifically, F preferably has askirt selectivity of at least about 24 db. per octave to minimizepossible undersired attenuation of targets at frequencies near the uppercutoff of F A preferred arrangement of F is then a two-section activehigh-pass filter with the feedback taken from the midpoint of the twosections. Such arrangement of F and F. provides good control of outputclutter residue when the input clutter to the filter has a spectrumtypical of that for non-coherent doppler radar. These and other suitablearrangements can also readily be implemented by one skilled in the art,in view of the teachings herein.

It is noted that a signal from a true moving target which is movingslowly may produce frequency components in the upper end of the passbandof filter F and that if these components are sufficiently strong theymay tend to actuate the clutter control circuit so as to raise thelower-frequency cutoff of filter F and interfere with the reception ofdesired signals from slower-moving targets. However, such interferingsignals are rarely strong enough to interfere with the detection ofdesired signals. As is the case with other elements in the system,adjustment and selection of parameter values for the clutter controlcircuit is such as to provide the desired reliability of response to thepresence or absence of moving-target signals, while still maintainingthe best possible sensitivity for a large range of target-objectvelocities. It is noted that certain probability of producing falsealarms can be tolerated in the system, since once an alarm is given theoperator can then direct his attention to determining whether a truemoving target is present or not.

FIG. 3 illustrates an arrangement of the system in which the noise AGCof the invention is utilized but the clutter control circuit is not.Parts corresponding to those of FIG. 1 are indicated by correspondingnumerals with the suffix A. Such an arrangement would be useful, forexample, in situations where only higher-velocity targets are ofinterest and therefor the low-frequency cutoff of filter F, can beraised to substantially completely eliminate clutter signals under alloperating conditions. The difficulty due to varying noise level wouldthen still remain, and would be compensated for by the use of the noiseAGC system.

FIG. 4 illustrates a system in which the noise AGC system is notutilized but the clutter control circuit of the invention is. Such anarrangement may be useful, for example, where the basic radar is of thecoherent, rather than the noncoherent type, and apparatus is includedtherein for maintaining the noise level constant. In this event,interference due to clutter is the primary problem in automaticdetection, and use of the clutter control circuit 14A mitigates theproblems due to clutter variations.

It is noted that when the clutter control circuit is used without thenoise AGC circuit it is possible to eliminate filter F,.

Other variations of construction and application of the invention willreadily appear to one skilled in the art in view of the foregoingteachings. Thus, while the invention has been described in the interestof definiteness with particular reference to specific embodimentsthereof, it may be embodied in any of a variety of forms diverse fromthose particularly described without departing from the scope of theinvention.

We claim:

I. In a noncoherent doppler radar system for detecting target objectsmoving with respect to reference objects by receiving and sensingsignals from said target objects having frequency componentsdoppler-shifted with respect to frequency components of signals fromsaid reference objects, said system comprising means for receiving saidsignals from said target objects and from said reference objects, firstvariable-gain means for amplifying said received signals from saidtarget objects and from said reference objects, first frequencyselectivemeans for selectively enhancing said doppler-shifted components withrespect to frequency components of said signal from said referenceobjects, automatic gain control means for controlledly varying the gainof said first amplifying means in response to the strength of signalsfrom said first amplifying means and in the sense to reduce the gainwhen said received signals increase in strength, the improvement whichcomprises:

noise AGC means, comprising second amplifying means supplied with theoutput of said first frequency-selective means, secondfrequency-selective means supplied with the output of said secondamplifying means for selectively passing noise frequency componentslying outside the spectra of frequency components produced by targetobjects and reference objects, means for generating a gaincontrol signalin response to noise frequency components passed by said secondfrequency-selective means, and means for varying the gain of said secondamplifying means in response to said control signal, in the sense tomaintain substantially constant the level of said noise frequencycomponents in said output of said second amplifying means; and

means supplied with the output of said second amplifying means fordetecting the presence or absence therein of said signals from targetobjects.

2. In a noncoherent doppler radar system comprising means fortransmitting time-spaced pulses of electrical energy, means forreceiving reflections of said transmitted pulses from reference objectsand from target objects moving with respect to said reference objects toproduce received signals, first variable-gain amplifier means foramplifying said received signals, gating means for selectively passingreceived signals produced by objects in different sequentially-selectedrange bins in response to different ones of said transmitted pulses,first AGC means responsive to the output of said gating means to varythe gain of said first variable-gain amplifier means in the sense toreduce said gain when the strength of said reflections from referenceobjects in the selected range bin increases; and first filter means forselectively enhancing frequency components in the output of said gatingmeans which are in or adjacent the spectral band produced by said targetobjects, the improvement comprising:

second variable-gain amplifier means supplied with the output of saidfirst filter means;

second AGC means responsive to those frequency components of noise inthe output of said second variablegain amplifier means which aresituated adjacent but outside said spectral band, for varying the gainof said second AGC means in the sense to maintain substantially constantthe level of said noise frequency components in the output thereof; and

means responsive to the output of said second AGC means for sensingchanges therein due to the presence or absence of signals due to targetobjects.

3. Automatic detection apparatus for use with a noncoherent dopplerradar of the type which produces a combined output signal comprisingclutter-signal frequency components of variable strength andfrequency-spread occupying a first spectral band, moving-target signalfrequency components in a second spectral band doppler-shifted upward infrequency with respect to said clutter-signal frequency components to anextent dependent upon the velocities of target objects, and noise-signalfrequency components of variable level and occupying a third spectralband extending within and above said second spectral band, saidapparatus comprising:

noise AGC means supplied with said combined output signal for producinga corresponding output signal in which said noise level is substantiallyconstant;

clutter-control means supplied with the output of said noise AGC meansfor automatically providing an increased attenuation of saidclutter-signal components in the higherfrequency portion of said firstspectral band when the energy of said last-named components increasesabove a predetermined level; and

energy-threshold means for producing an output signal in response toincreases above a threshold value of the energy of the output-from saidclutter-control means.

4. Apparatus in accordance with claim 3, in which said noise AGC meanscomprises variable-gain means, and first filter means selectivelyresponsive to noise frequency components outside said second spectralband for developing and applying to said variable-gain means again-control signal to maintain said noise level substantially constantin the output of said variable-gain means; and

in which said clutter-control means comprises second filter means havinga lower-frequency skirt which is controllably variable in frequencyposition in the higher-frequency portion of said first spectral band andthird filter means for selecting frequency components in saidhigherfrequency portion of said first spectral band for producing andapplying to said second filter means a control signal to vary thefrequency position of said skirt.

1. In a noncoherent doppler radar system for detecting target objectsmoving with respect to reference objects by receiving and sensingsignals from said target objects having frequency componentsdoppler-shifted with respect to frequency components of signals fromsaid reference objects, said system comprising means for receiving saidsignals from said target objects and from said reference objects, firstvariable-gain means for amplifying said received signals from saidtarget objects and from said reference objects, firstfrequency-selective means for selectively enhancing said doppler-shiftedcomponents with respect to frequency components of said signal from saidreference objects, automatic gain control means for controlledly varyingthe gain of said first amplifying means in response to the strength ofsignals from said first amplifying means and in the sense to reduce thegain when said received signals increase in strength, the improvementwhich comprises: noise AGC means, comprising second amplifying meanssupplied with the output of said first frequency-selective means, secondfrequency-selective means supplied with the output of said secondamplifying means for selectively passing noise frequency componentslying outside the spectra of frequency components produced by targetobjects and reference objects, means for generating a gain-controlsignal in response to noise frequency components passed by said secondfrequency-selective means, and means for varying the gain of said secondamplifying means in response to said control signal, in the sense tomaintain substantially constant the level of said noise frequencycomponents in said output of said second amplifying means; and meanssupplied with the output of said second amplifying means for detectingthe presence or absence therein of said signals from target objects. 2.In a noncoherent doppler radar system comprising means for transmittingtime-spaced pulses of electrical energy, means for receiving reflectionsof said transmitted pulses from reference objects and from targetobjects moving with respect to said reference objects to producereceived signals, first variable-gain amplifier means for amplifyingsaid received signals, gating means for selectively passing receivedsignals produced by objects in different sequentially-selected rangebins in response to different ones of said transmitted pulses, first AGCmeans responsive to the output of said gating means to vary the gain ofsaid first variable-gain amplifier means in the sense to reduce saidgain when the strength of said reflections from reference objects in theselected range bin increases; and first filter means for selectivelyenhancing frequency components in the output of said gating means whichare in or adjacent the spectral band produced by said target objects,the improvement comprising: second variable-gain amplifier meanssupplied with the output of said first filter means; second AGC meansresponsive to those frequency components of noise in the output of saidsecond variable-gain amplifier means which are situated adjacent butoutside said spectral band, for varying the gain of said second AGCmeans in the sense to maintain substantially constant the level of saidnoise frequency components in the output thereof; and means responsiveto the output of said second AGC means for sensing changes therein dueto the presence or absence of signals due to target objects. 3.Automatic detection apparatus for use with a noncoherent doppler radarof the type which produces a combined output signal comprisingclutter-signal frequency components of variable strength andfrequency-spread occupying a first spectral band, moving-target signalfrequency components in a second spectral band doppler-shifted upward infrequency with respect to said clutter-signal frequency components to anextent dependent upon the velocities of target objects, and noise-signalfrequency components of variable level and occupying a third spectralband extending within and above said second spectral band, saidapparatus comprising: noise AGC means supplied with said combined outputsignal for producing a corresponding output signal in which said noiselevel is substantially constant; clutter-control means supplied with theoutput of said noise AGC means for automatically providing an increasedattenuation of said clutter-signal components in the higher-frequencyportion of said first spectral band when the energy of said last-namedcomponents increases above a predetermined level; and energy-thresholdmeans for producing an output signal in response to increases above athreshold value of the energy of the output from said clutter-controlmeans.
 4. Apparatus in accordance with claim 3, in which said noise AGCmeans comprises variable-gain means, and first filter means selectivelyresponsive to noise frequency components outside said second spectralband for developing and applying to said variable-gain means again-control signal to maintain said noise level substantially constantin the output of said variable-gain means; and in which saidclutter-control means comprises second filter means having alower-frequency skirt which is controllably variable in frequencyposition in the higher-frequency portion of said first spectral band andthird filter means for selecting frequency components in saidhigher-frequency portion of said first spectral band for producing andapplying to said second filter means a control signal to vary thefrequency position of said skirt.